This question seems to fit Dill perfectly. Charles Baker Harris is an undersized "curiosity" who hails from Meridian, Mississippi. He comes to stay with his Aunt Rachel Haverford each summer and living next door to the Finches, he quickly becomes best friends with Jem and Scout. Dill takes to Maycomb life and he soon becomes their third partner in mischief. Dill is an unhappy little boy: It is apparent that his parents prefer life without Dill since according to Scout's Cousin Francis,
"... he just gets passed around from relative to relative, and Miss Rachel keeps him every summer." (Chapter 9)
Dill escapes his troubles at home with an active imagination, and his stories at first make Jem and Scout envious of his worldliness. But they soon see that most of Dill's stories are lies, and Scout recognizes that
Dill Harris could tell the biggest ones I ever heard. Among other things, he had been up in a mail plane seventeen times, he had been to Nova Scotia, he had seen an elephant, and his granddaddy was Brigadier General Joe Wheeler and had left him his sword." (Chapter 5)
Dill's "whoppers" are meant to impress his new friends, but he doesn't yet realize that Jem and Scout admire him for other reasons: his intelligence, sincerity and curious imagination among them. Dill cries when he has to leave Maycomb after his second summer: He has asked Scout to marry him, and he "kissed me swiftly in front of Jem" after their adventure in the Radley's back yard. Things get worse for Dill before they get better--so bad that he runs away from home and shows up under Scout's bed during Part Two of the novel.